Ubiquitin is a small, 76 amino acid proteins that is a post-translational modification (PTM) attached to protein targets. It is central to eukaryotic biology and controls protein degradation, trafficking, and regulation. Problems within the ubiquitin regulatory system can stem from or cause disease. However, unlike other PTMs, ubiquitin can be subsequently modified, creating chains of varying linkage and size that give rise to the wide array of biological effects of target proteins. This complexity is sometimes referred t as the ubiquitin code. The study of poly-Ub chains has historically been hindered by access to these molecules. This proposal capitalizes on recent developments in unnatural amino acid mutagenesis and protein chemistry to synthesize poly-Ubs. The proposed research will i) synthesize and immobilize non-canonical Ub chains, ii) use a proteomics strategy to enrich and identify novel cellular Ub-binding proteins, and iii) use phage display to identify high-affinity ply-Ub binders. These three goals will provide drastically improved understanding of the ubiquitin code and provide research tools to all researchers working in ubiquitin biology.